This application claims the priority of Japanese Patent Applications Nos. 2001-255846 filed on Aug. 27, 2001 and 2001-299228 filed on Sep. 28, 2001, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electronic endoscope with a power scaling function and, more particularly, to power scaling operation control of an electronic endoscope capable of observing an optically magnified image by a movable lens and forming an electronically magnified image through signal processing.
2. Description of the Related Art
An electronic endoscope, etc. in recent years has an objective lens system at the tip of its scope provided with a power scaling movable lens and drives this movable lens using an actuator, etc. to optically magnify an image of an object under observation. This optically enlarged image is picked up by an image pick-up element such as a CCD (Charge Coupled Device), and various types of image processing are performed by a processor based on the output signal from the CCD, by which an enlarged image of a subject to be observed is displayed on a monitor. In such an optical variable power mechanism, an observed image can be enlarged up to about 70 to 100 times.
On the other hand, conventionally, the image obtained by the CCD is enlarged electronically by picture element interpolation processing etc. of an electronic variable power circuit. According to this, the optically enlarged image can be further enlarged and displayed on the monitor for observation.
The power scaling function of such an electronic endoscope electronically magnifies an image which has been optically magnified under an arbitrary magnification using optical power scaling and electronic power scaling switches respectively or operates by associating optical power scaling with electronic power scaling using a common power scaling switch of an endoscope operation section. When this common power scaling switch is used, the movable lens is moved to the magnification end (near end) by optical power scaling, and then optical power scaling is automatically changed to electronic power scaling to form a further magnified image through signal processing, which makes it possible to observe specific areas such as affected areas speedily and under an optimal magnification.
However, the optical power scaling mechanism using a movable lens in the above-described conventional electronic endoscope has a smaller depth of field as the magnification increases and there are cases where the electronic endoscope cannot optimally display an overall image of an object under observation with an uneven surface in the depth direction. This phenomenon will be described now with reference to FIGS. 12 and 13.
In FIG. 12, the left-hand side view shows a state in which when a movable lens 1 lies at the Far end, the proximal end, a subject to be observed 2 forms an image on a CCD image pick-up surface 3, and the right-hand side view shows an image formation state at the time when the moving lens 1 is moved to the Near side, the enlargement side. In FIG. 12, since the moving lens 1 is set at a position of distance 0, at the time of enlargement, the image pick-up surface 3 is drawn so as to be shifted rearward. Actually, the moving lens 1 moves forward. When the optical enlargement is not made as shown in the left-hand side view of FIG. 12, the focus is sharpened, for example, at a distance of 8 to 100 mm, and the depth of field is 92 mm. Whereas, when the optical enlargement is made as shown in the right-hand side view in FIG. 12, the focus is sharpened at a distance of 4 to 20 mm, and the depth of field is 16 mm.
FIG. 13 is an explanatory view of the depth of field. Taking the focal length of a lens 4 as f, the F number as FN, the allowable blur circle as xcex4, and the distance of subject to be observed as L, the rear depth of field Lr and the front depth of field Lf are expressed as follows:
Lr=(xcex4xc2x7FNxc2x7L2)/(f2xe2x88x92xcex4xc2x7FNxc2x7L)xe2x80x83xe2x80x83(1) 
Lf=(xcex4xc2x7FNxc2x7L2)/(f2+xcex4xc2x7FNxc2x7L)xe2x80x83xe2x80x83(2) 
The depth of field of this lens 4 is a value obtained by summing up the rear depth of field Lr and the front depth of field Lf, that is, Lr+Lf. The depth of focus is 2xcex4xc2x7FN.
The aforementioned depth of field explained in FIG. 12 is also the above-described value of Lr+Lf, and the range in which the focus is sharp is 92 mm at the Far end and 16 mm at the Near end. In the configuration of variable power objective optical system now used for an endoscope, the depth of field decreases as the image is enlarged. Therefore, in the case where a subject to be observed having irregularities is observed, the depth of field becomes shallow (short), so that a blur occurs somewhere in the depth direction. When the subject to be observed caught in a state of shallow depth of field is enlarged electronically, the blur in the depth direction is also enlarged, which presents a problem in that the whole of the subject to be observed cannot be displayed and observed with high picture quality.
The present invention has been implemented in view of the above-described problem and it is an object of the present invention to provide an electronic endoscope having a power scaling function capable of eliminating blurring in the depth direction by switching only the depth of field to an optimal value and smoothly observing an image.
To attain the above-described object, the present invention is characterized by including an objective optical system that optically scales power of an image under observation using a power scaling lens, an electronic power scaling circuit that electronically scales the image obtained through an image pickup element through signal processing, depth operating means that changes the depth of field at the optical power scaling to an arbitrary value and a control circuit that drives and controls the objective optical system so that the objective optical system is set to the depth of field selected by the operation of this depth operating means and controls the electronic power scaling operation of the electronic power scaling circuit so that it maintains the magnification of the image immediately before the depth operation.
Since the depth of field is specified by the position of the movable lens to magnify the image, the depth of field can also be recognized by the optical magnification. According to the above-described configuration, assuming that a predetermined depth of field (e.g., depth equivalent to xc3x9760) is selected by the depth operating means when an image is magnified to xc3x9772, for example, through optical power scaling, xc3x971.2 is set by electronic power scaling. This makes it possible to display a magnified image under the same magnification as that immediately before the depth operation. Furthermore, assuming that the depth of field value is 7 mm under xc3x9772 and 12 mm under xc3x9760 as described above, it is possible to widen the sharply focused range 5 mm in the depth direction and observe a magnified image focused within a desired range.
Furthermore, the control circuit can be controlled so that it does not electronically scale power beyond a preset allowable magnification range when the depth of field is changed to an arbitrary value.
The depth operating means can be constructed by including a selection switch that selects a plurality of preset depth of field values and a variable switch to change the depth of field values selectable by this switch. Furthermore, this depth operating means can also be made to increase the depth of field from the immediately preceding value by a predetermined amount when a depth of field change operation is performed.
Another embodiment of the present invention is characterized by including an objective optical system that optically scales an image under observation using a power scaling lens, an electronic power scaling circuit that electronically scales the image obtained through an image pickup element through signal processing, depth operating means that presets a comparative observation depth of the depth of field which changes through the optical power scaling operation and a control circuit that drives and controls, when a predetermined operation is performed during execution of optical power scaling, the objective optical system so that the objective optical system is set to the comparative observation depth set by the depth setting means and controls the electronic power scaling operation of the electronic power scaling circuit so that the magnification of the image with the comparative observation depth taken by the image pickup element is identical to the magnification immediately before the depth operation.
In the above-described embodiment, assuming that the predetermined operation is an operation of displaying a still image of a freeze switch, it is possible to simultaneously display the still image during an operation of the freeze switch and the still image of the comparative observation depth on a partitioned screen on the monitor.
According to the above-described other embodiment, the depth setting means can preset, for example, three (one or a plurality of) comparative observation depths and when a predetermined operation member (e.g., the freeze switch) is turned ON while optical power scaling is set to a magnification of xc3x97A, for example, a xc3x97A image is displayed on one small screen of the 4-partitioned screen and at the same time, three images obtained by setting the power scaling movable lenses to three comparative observation depths one by one are displayed on the rest of the small screens of the 4-partitioned screen under the same magnification xc3x97A. That is, when the depth of field changes, the optical magnification also changes, and therefore compensating for this change by electronic magnification processing allows an image to be formed under the same magnification.
For example, suppose the depth setting means has set comparative observation depths of 12 mm, 24 mm and 36 mm. These depths can be specified at the drive positions of the movable lens and also recognized by the power of optical magnification and the comparative observation depths are equivalent to xc3x9760, xc3x9750 and xc3x9740 in that order. Assuming that the magnification xc3x97A is xc3x9756, the power of electronic power scaling is set to approximately xc3x970.933 for an image with a depth of field of 12 mm, xc3x971.12 for an image of 24 mm, and xc3x971.4 for an image of 36 mm. As a result, it is possible to display four magnified images with different focused depth ranges under the same magnification.